I use protection diodes on the front end of my W1FB preamps (as well as
before an IC amp in the preamp, and on the output of the preamp) and have
never been able to detect receiver performance degradation on 160 meters
due to the clamping diodes (had to include the diode clamps before the IC
amp in the W1FB preamp to prevent continuous failures of this IC when
transmitting).
What works OK in one place will almost always not work the same in another.
Intermod is a function of the TOTAL voltage of all the signal sources at the
point where the diodes are. This is why some systems work with diodes, and
some systems completely fall apart. The problem can come from a hundred
small signals, each signal far below threshold, or just one large signal
near threshold. Impedance also matters, both in band and out-of-band.
A regular diode is about .5 volts or before peak it starts to generate junk,
some are less and some slightly more.
Assuming .5 volts peak, that would be .35 volts RMS.
.35V RMS is only 4 dBm at 50 ohms. This would be the peak composite voltage
level, not the single tone or composite power calculated from the simple sum
of powers. This paper would apply:
www.minicircuits.com/app/AN60-037.pdf
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